The present invention relates to a copier, facsimile apparatus, printer or similar electrophotographic image forming apparatus and, more particularly, to a developing device for an image forming apparatus, and a toner cartridge.
A latent image electrostatically formed on an image carrier is usually developed by either a method using toner, or single-ingredient type developer, or a mixture of toner and carrier or two-ingredient type developer. Typical of this kind of developing method is a magnet brush developing method as disclosed in, e.g., U.S. Pat. No. 2,874,063. In the two-ingredient type developer, fine toner particles are retained on the surfaces of comparatively great magnetic carrier particles due to an electrostatic force generated by friction. When this type of developer approaches a latent image, the force of an electric field formed by the latent image and attracting the toner toward the latent image overcomes the force coupling the toner and carrier. As a result, the toner develops the latent image to produce a corresponding toner image. The prerequisites with the method using the two-ingredient type developer is that fresh toner be replenished in order to make up for consumption and to maintain the toner and carrier in a constant mixture ratio, i.e., to maintain a toner concentration constant. These prerequisites cannot be met without resorting to a toner replenishing mechanism, toner concentration sensor, and so forth, resulting in a bulky developing device and complicated mechanisms.
In light of the above,. Japanese Patent Publication No. 5-67233, for example, teaches a developing device using a two-ingredient type developer and eliminating the need for toner concentration control. In this developing device, a developer around a developer carrier automatically takes in fresh toner at a toner replenishing position. A regulating member charges the toner while regulating the amount of the developer. Therefore, the device is capable of charging the toner while maintaining the toner concentration of the developer constant without resorting to a toner replenishing mechanism or a toner concentration sensor.
It has been customary with an electrophotographic image forming apparatus to uniformly charge a photoconductive element or image carrier, illuminate the surface of the charged element with imagewise light to thereby form a latent image, develop the latent image with toner fed from a developing device, transfer the resulting toner image from the element to a paper, and then fix the toner image on the paper. In this type of apparatus, means for replenishing the toner to the developing device is often implemented as a toner cartridge removably mounted to, e.g., a toner tank included in the developing device. Japanese Patent Laid-Open Publication No. 60-41068, for example, proposes a toner cartridge whose outlet is closed by a seal member when the cartridge is not used. This kind of cartridge is mounted to an image forming apparatus after the seal has been removed from the cartridge. Also, Japanese Patent Laid-Open Publication No. 60-21070 teaches an arrangement wherein a cover slidable in the same manner as a cover for covering a developing roller, or developer carrier, selectively opens or closes an outlet formed in a toner cartridge.
The apparatus disclosed in the above Patent Publication No. 5-67233 has some problems yet to be solved, as follows. In order to desirably transfer the fresh toner to the developer carried on the developer carrier, the amount of developer cannot be increased, compared to a conventional apparatus using a two-ingredient type developer. Hence, when the apparatus is applied to a high-speed apparatus having a developer carrier whose surface moves at a high linear velocity, the toner cannot be sufficiently charged and is, therefore, apt to contaminates the background of an image. When the regulating stress of the regulating member is increased in order to sufficiently charge the toner, the developer particles impinge on each other and generate heat. The heat causes the toner to form films on the surfaces of the magnetic particles, thereby causing the developer to be spent. As a result, the charging characteristic of the magnetic particles is sequentially deteriorated to such a degree that the toner flies about and contaminates the background.
On the other hand, development using a single-ingredient type developer or toner causes the toner to deposit on the surface of a developer carrier due to an electrostatic force generated by friction between the toner and the developer carrier, or due to a magnetic force generated between the toner and the developer carrier. Of course, for the magnetic force scheme, use is made of toner containing a magnetic substance, and a developer carrier accommodating magnets therein. When the toner approaches a latent image, a force generated by an electric field formed by the latent image and attracting the toner toward the latent image overcomes a force coupling the toner and developer carrier. As a result, the toner develops the latent image. This kind of development does not have to control the toner concentration and, therefore, reduces the size of the developing device. However, because the number of toner particles in a developing region is smaller than the number available with the two-ingredient type development, the amount of toner to deposit on the latent image is too small for the device to be applied to a high-speed apparatus.
In order to achieve a miniature developing apparatus using a two-ingredient type developer, an arrangement may be made such that toner is circulated around and along a developer carrier together with a carrier. This allows the toner to be charged by friction and introduced into the developer. However, the prerequisite with this arrangement is that a relatively small amount of carrier be deposited on the developer carrier in order to insure the migration of the toner into the developer. The life of a developer is proportional to the amount of carrier, as well known in the art. When the amount of carrier deposited on the developer carrier is small, as mentioned above, the deterioration of the developer is accelerated due to repeated agitation, circulation, or conveyance. This, coupled with the noticeable melting of the toner, results short charging and thereby reduces the life of the developer.
The toner cartridge and image forming apparatus operable therewith as taught in the previously mentioned Laid-Open Publication No. 60-41068 has the following drawbacks. When the seal member is removed from the cartridge to be mounted to the apparatus or is disposed of later, the toner is apt to smear the operator's hand, clothing or the like even if the operator handles the cartridge with care. This is also true with the cover scheme of the Laid-Open Publication No. 60-21070. Specifically, when the cover is opened, the toner deposited on the rear of the cover is transferred to the cartridge and therefrom to the operator's hand, closing or the like.